FIGS. 1(a), 1(b) and 1(c) are views for explaining a general principle of a capacitor-type microphone which is one type of vibration sensors. A capacitor-type microphone 11 has a structure in which an opposing electrode plate 12 and a vibration electrode plate 13 are aligned face to face with each other with a small gap, with a dc voltage being applied between the two electrode plates 12 and 13 by a dc power supply 14. The opposing electrode plate 12 is allowed to have sufficient rigidity, or secured so as not to vibrate, and the vibration electrode plate 13 is made thinner in thickness so as to be vibrated by sound vibrations.
In this structure, when a sound vibration is transmitted to the capacitor-type microphone 11 as shown in FIG. 1(a), the thin vibration electrode plate 13 is vibrated by the sound vibration as shown in FIG. 1(b) to cause the electrostatic capacity between the opposing electrode plate 12 and the vibration electrode plate 13 to change. By electrically detecting the change in the electrostatic capacity, it is possible to extract sound (change in sound pressure) as shown in FIG. 1(c).
FIG. 2 is a cross-sectional view that shows a structure of a conventional capacitor-type microphone manufactured by utilizing a micro-machining technique. This capacitor-type microphone 21 has a structure in which: the upper face of a silicon substrate 22 having a through hole 27 opened in the center is covered with an insulating film 23, and a vibration electrode plate 24 is formed on the through hole 27, with an opposing electrode plate 26 being formed on the lower face of a perforated member 25 that covers the upper side of the vibration electrode plate 24. Thus, in the capacitor-type microphone 21, when a sound vibration is directed therein through the holes of the perforated member 25 and the opposing electrode plate 26 or through the through hole 27 in the lower face, to cause the vibration electrode plate 24 to vibrate, the electrostatic capacity between the vibration electrode plate 24 and the opposing electrode plate 26 is changed so that the sound vibration is outputted as a change in the electrostatic capacity.
In manufacturing processes of this capacitor-type microphone 21, after the insulating film 23, the vibration electrode plate 24 and the like have been formed on the upper face of the silicon substrate 22, the through hole 27 is opened by etching the silicon substrate 22 from the lower face side. With respect to the silicon substrate 22, in general, a (100) plane silicon wafer is used because it is easily available at a comparatively low price. For this reason, when the silicon substrate 22 is etched from the back face side, a plane having [111] orientation or an orientation equivalent to this, which is a dense plane of (100) plane silicon substrate, appears in the through hole 27 to cause a tilted face, with the result that the through hole 27 having a truncated pyramid shape is formed in the silicon substrate 22. Moreover, since the silicon substrate 22 is etched from the lower face side, the through hole 27 has a larger width on the lower face side of the silicon substrate 22 and a narrower width on the upper face side thereof.
For this reason, the opening area on the lower face side of the through hole 27 becomes larger than the area of the actual vibration portion of the vibration electrode plate 24 to cause the area of the silicon substrate 22 to become larger correspondingly. As a result, the conventional structure makes it difficult to miniaturize the capacitor-type microphone 21. Here, in the case when the thickness of the silicon substrate 22 is made thinner, although the opening area ratio between the upper face side and the lower face side of the through hole 27 becomes close to 1, there is a limitation in making the thickness of the silicon substrate 22 thinner from the viewpoint of the strength of the silicon substrate 22.
Moreover, Patent Document 1 has disclosed a piezo-resistor-type pressure sensor that detects a pressure of air or the like by converting a positional change of a thin film portion formed on a semiconductor substrate to a change in resistance value. In this piezo-resistor-type pressure sensor, in order to solve the above-mentioned problem caused by forming the thin film portion by etching the semiconductor substrate from the lower face side, the semiconductor substrate is etched from the upper face side to form a thin film portion. With this arrangement, after carrying out a film-forming process on the semiconductor substrate (silicon wafer) to form a thin film portion, an opening section is formed on a part of the thin film portion so that the silicon wafer is exposed, and an isotropic etching process is carried out through this opening section to provide a cavity in the semiconductor substrate so that the thin film portion is supported in a floating state from the upper face of the silicon substrate.
However, in the case of the microphone that is not used for measuring the absolute pressure of air, but is necessary to acquire sound as a smaller air pressure variation, the piezo resistor system composed of one thin film tends to cause problems with hysteresis and the like. For this reason, in general, an electrostatic capacitive system composed of two thin films is adopted. Even in this case, the structure having a rectangular shaped opening section or a square-frame-shaped opening section as disclosed in Patent Document 1 fails to form a thin film portion (vibration electrode plate) having superior sensitivity and frequency characteristics suitable for the microphone.    Patent Document 1: Japanese Patent Application Laid-Open No. 9-82983    Patent Document 2: Japanese Patent Application National Publication (Laid-Open) No. 2004-506394    Patent Document 3: Japanese Patent Application Laid-Open No. 2004-128957    Patent Document 4: Japanese Patent Application Laid-Open No. 2002-27595    Patent Document 5: Japanese Patent Application Laid-Open No. 62-284233    Patent Document 6: Japanese Patent Application National Publication (Laid-Open) No. 9-508777    Patent Document 7: Japanese Patent Application Laid-Open No. 2001-13156